DE102014203040A1 - Illumination system of a microlithographic projection exposure apparatus and method for operating such - Google Patents

Abstract

An illumination system of a microlithographic projection exposure apparatus (10) has a light source (LS) which is set up to generate a sequence of light pulses. In the light path between the light source (LS) and a target surface (38) an array (28) of optical elements (30) is arranged, which are digitally switchable between two switching positions. An adjustable light deflection optic (32) arranged in the light path between the array (28) and the target surface (38) is arranged to deflect incident light at different deflection angles. A control device (52) controls the light deflection optics (32) in such a way that a change in the deflection angle takes place between two successive light pulses.

Description

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention relates to an illumination system of a microlithographic projection exposure apparatus which comprises a pulsed light source and a DMD (digital mirror device) or another array of optical elements which can be digitally switched between two switching positions.

2. Description of the Related Art

Integrated electrical circuits and other microstructured devices are typically fabricated by applying a plurality of patterned layers to a suitable substrate, which is typically a silicon wafer. To pattern the layers, they are first covered with a resist that is resistant to light of a certain wavelength range, e.g. Light in the deep ultraviolet (DUV, deep ultraviolet), vacuum ultraviolet (VUV, vacuum ultraviolet) or extreme ultraviolet (EUV, extreme ultraviolet) spectral range is sensitive. Subsequently, the thus coated wafer is exposed in a projection exposure apparatus. In this case, a pattern of diffractive structures, which is arranged on a mask, is imaged onto the photoresist with the aid of a projection objective. In general, since the magnification amount is smaller than 1, such projection lenses are sometimes referred to as reduction lenses.

After developing the photoresist, the wafer is subjected to an etching process, whereby the layer is patterned according to the pattern on the mask. The remaining photoresist is then removed from the remaining parts of the layer. This process is repeated until all layers are applied to the wafer.

In the prior art lighting systems are known which use mirror arrays in order to illuminate the pupil plane of the illumination system variable. Examples of this can be found in the EP 1 262 836 A1 . US 2006/0087634 A1 . US 7,061,582 B2 . WO 2005/026843 A2 and WO 2010/006687 A1 , In general, these are mirror arrays in which the mirrors can be tilted continuously over a certain angle range.

From the WO 2012/100791 A1 For example, an illumination system is known which additionally has a digitally switchable micromirror array. This micromirror array is imaged by means of a lens onto the light entrance facets of an optical integrator. A similar, but differently controlled lighting system is from the filed on 22.11.2013 European patent application with the file number EP 13194135.3 entitled "Illumination System of a Microlithographic Projection Exposure Apparatus", the contents of which are hereby incorporated by reference.

When using digitally switchable micromirror arrays in lighting systems, the number of micromirrors needed is generally so high that it can not be provided by a single micromechanical component. Therefore, in the already mentioned European patent application with the file number EP 13194135.3 proposed to arrange a plurality of smaller micromirror arrays side by side on a support and to image with the aid of a special imaging optics on the target surface, that the images of the individual arrays adjoin one another seamlessly on the target surface.

Similar problems arise when, in the illumination system, the array does not contain micromirrors, but other switchable optical elements, e.g. Liquid crystal cells, as used in LCDs.

Another problem with the use of arrays of switchable optical elements is that occasionally the optical elements are relatively widely spaced. If such an array is imaged on a target surface, correspondingly large gaps are created between the images of the optical elements, which is often undesirable.

From the US 6,624,880 B2 , of the US 2011/0134407 A1 and the US 7,957,055 B2 Projection exposure systems are known in which a lighting system does not illuminate conventional masks, but a micromirror array that takes over the function of the mask. Such systems are often referred to as "maskless".

The US 2011/0240611 A1 describes a projection exposure apparatus in which a one-dimensional micromirror array is used to ablate material directly on the wafer.

SUMMARY OF THE INVENTION

The object of the invention is to specify an illumination system of a microlithographic projection exposure apparatus with which a larger target area can illuminate uniformly in a particularly simple manner with the aid of an array of optical elements which can be digitally switched between two switch positions.

According to the invention, this object is achieved by an illumination system of a microlithographic projection exposure apparatus which has a light source which is set up to generate a (in particular periodic) sequence of light pulses. In the light path between the light source and a target surface, an array of optical elements is arranged, which are digitally switchable between two switching positions. Arranged in the light path between the array and the target surface is an adjustable light deflecting optic arranged to deflect incident light at different deflection angles. A control device controls the light deflection optics so that a change in the deflection angle between two successive light pulses takes place.

According to the invention, the target area is thus never fully illuminated at any given time, but only partially. The relatively long times between successive pulses of light are used to adjust the light deflection optics and, if necessary, also to switch the optical elements of the array to another switching position. In this way, the desired light distribution on the target surface is built up successively similar to the time division multiplexing. The overall intensity distribution then arises through integration over time. By switching at least one of the optical elements of the array between two consecutive light pulses, a different light pattern can be generated on each area illuminated in the target area.

In illumination systems, such a successive build-up of an intensity distribution on a target surface is generally tolerable since the photoresist also integrates the intensity incident thereon. The photoresist does not change its chemical properties until the intensity integrated during the entire exposure (sometimes also called dose) exceeds a certain threshold.

In an illumination system, therefore, the illumination angle distribution in the mask plane (and thus also in the image planes of the projection objective that are optically conjugate thereto) is predetermined by the temporally integrated spatial distribution of the projection light in the pupil surface. If the target area illuminated by the array is the pupil area or a surface conjugate thereto or at least determines its illumination, then the illumination angle distribution can be successively combined during the exposure from a plurality of individual distributions by the inventive successive illumination of the pupil area.

The adjustable light deflection optics may be any optical element that can deflect incident light in different directions. Reflective optical elements, such as tiltable planar mirrors or rotating polygon mirrors, or refractive optical elements, such as tiltable or rotating wedge elements, are particularly suitable.

In one embodiment, the light deflecting optics and the controller are arranged such that a first light pattern that is illuminable on the target surface prior to a change in the deflection angles does not overlap with a second light pattern that is illuminable on the target surface after the deflection angles have changed. In this way it is ensured that a maximum target area can be covered by the successive illumination of the light field with a predetermined number of light pulses. Under a light pattern in this context, the amount of all illuminated areas understood; the dark areas remaining between the illuminated areas are thus not part of the light pattern.

It is contemplated to arrange the first light pattern and the second light pattern on the target field so that envelopes (i.e., outer contours) of the two light patterns do not overlap. The two light patterns can thus be arranged, for example, in a row next to one another or in a two-dimensional arrangement in a predetermined grid on the target surface. In this way, larger target areas can be illuminated by the array very quickly. Such control of the light deflecting optics and the optical elements of the array is particularly useful when unavoidable gaps between the optical elements are not transferred to the target surface or are so small that they can be tolerated.

On the other hand, if such gaps are intolerably large, it is often better to interlock the first light pattern and the second light pattern. The light deflection optics then only slightly offset the light patterns between two light pulses, so that the envelopes of the light patterns overlap considerably. Although the light patterns themselves overlap, virtually any (even multi-level) intensity distributions on the target surface can be illuminated by the array after time integration.

This is particularly expedient if the distance between two adjacent optical elements of the array along a reference direction is greater than the maximum dimension of the two optical elements along the reference direction. The first and the second light pattern can then be interlaced so that the second Light pattern on the target area illuminates gaps that can not be illuminated by the first light pattern. Such gaps can arise, in particular, when a lens is arranged in the light path between the array and the target surface, which images the array onto the target surface.

The light source may be a laser configured to produce projection light having a center wavelength of 150 nm and 250 nm.

• a) Richten einer Abfolge von Lichtpulsen auf ein Array von optischen Elementen, die zwischen zwei Schaltstellungen digital umschaltbar sind, wobei das Array eine Zielfläche ausleuchtet;
• b) Ablenken des von dem Array auf die Zielfläche gerichteten Lichts um unterschiedliche Ablenkwinkel durch eine im Lichtweg zwischen dem Array und der Zielfläche angeordnete verstellbare Lichtablenkungsoptik, wobei eine Änderung der Ablenkwinkel zwischen zwei aufeinander folgenden Lichtpulsen erfolgt.

The invention also provides a method for operating an illumination system of a microlithographic projection exposure apparatus, comprising the following steps:

• a) directing a sequence of light pulses onto an array of optical elements that are digitally switchable between two switch positions, the array illuminating a target area;
• b) deflecting the light directed from the array onto the target surface by different deflection angles through an adjustable light deflection optic arranged in the light path between the array and the target surface, wherein a change in the deflection angles occurs between two consecutive light pulses.

The advantages of the method according to the invention essentially correspond to those which have been explained above in connection with the illumination system according to the invention.

At least one of the optical elements of the array can switch between two light pulses. As a result, a wide variety of intensity distributions can be generated on the target surface after temporal integration.

A first light pattern illuminated before a change in the deflection angle of the array on the target surface preferably does not overlap with a second light pattern illuminated on the target surface after the deflection angle has changed from the array.

The first light pattern and the second light pattern may be arranged on the target field such that envelopes of the two light patterns do not overlap.

Alternatively, the first light pattern and the second light pattern may be interlaced. This is particularly useful when the distance between adjacent optical elements of the array along a reference direction is greater than the maximum dimension of an optical element along the reference direction.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparent from the following description of exemplary embodiments with reference to the drawings. Show:

1 a highly simplified perspective view of a microlithographic projection exposure apparatus;

2a to 2 B Parts of an illumination system according to the invention in a schematic perspective view at different times during a scan;

3 a part of a lighting system according to the invention according to another embodiment in one of the 2a to 2d ajar representation in which a micromirror array is imaged by a lens on the target surface;

4 a schematic representation for explaining an embodiment in which the light pattern generated by the micromirror array in succession on a mirror surface are interlocked.

DESCRIPTION OF PREFERRED EMBODIMENTS

The 1 shows a highly schematic perspective view of a projection exposure system 10 , which is suitable for the lithographic production of microstructured components. The projection exposure machine 10 includes a light source LS adapted to produce projection light having a center wavelength of 193 nm and a lighting system 12 which transmits the projection light generated by the light source LS onto a mask 14 directed and there a narrow, rectangular in the illustrated embodiment illumination field 16 illuminates. Other illumination field shapes, eg ring segments, are also possible.

Inside the lighting field 16 underlying structures 18 on the mask 14 be using a projection lens 20 comprising a plurality of lenses L1 to L4 on a photosensitive layer 22 displayed. The photosensitive layer 22 , which may be, for example, a photoresist, is on a wafer 24 or another suitable substrate and is located in the image plane of the projection lens 20 , Because the projection lens 20 in general a magnification | β | <1 has, within the illumination field 16 lying structures 18 reduced to a projection field 18 ' displayed.

In the illustrated projection exposure system 10 become the mask 14 and the wafer 24 during the projection proceed along a direction marked Y. The ratio of the travel speeds is equal to the magnification β of the projection lens 20 , If the projection lens 20 the image inverted (ie β <0), the traversing movements of the mask run 14 and the wafer 24 in reverse, as in the 1 is indicated by arrows A1 and A2. In this way, the lighting field 16 in a scanning motion over the mask 14 Guided so that even larger structured areas contiguous to the photosensitive layer 22 can be projected.

The 2a shows parts of the lighting system 12 in a schematic perspective view. On a carrier 26 is a micromirror array 28 arranged, which has a variety of micromirrors 30 contains. Every micromirror 30 is digitally switchable between two switch positions. The micromirrors 30 are in a regular two-dimensional arrangement over the surface of the micromirror array 28 distributed.

In the micromirror array 28 it may in particular be a DMD (digital mirror device), which is usually realized as a micro-electromechanical component (MEMS, micro-electromechanical system).

In the light path behind the micromirror array 28 is a tilting mirror 32 arranged by means of not shown actuators around two orthogonal tilt axes 34 . 36 can be tilted. The tilting mirror 32 is located in the optical light path between the micromirror array 28 and a target area 38 which are, for example, the light entrance facets of an optical integrator or a pupil surface of the illumination system 12 can act.

In the 2a is the tilting mirror 32 shown in a first tilted position. In this first tilt position is that of the micromirror array 28 reflected projection light 40 so on the target area 38 directed that there in a first area 42a a light pattern 44a arises. The light pattern 44a depends on the position of the micromirrors 30 in the micromirror array 28 from. By suitable control of the micromirrors 30 can thus the light pattern 44a in the first area 42a to be changed. The more micromirror 30 in the micromirror array 28 are included, the greater is the spatial resolution with which the light pattern 44a can be set.

When operating the projection exposure system becomes the light pattern 44a in the first area 42a during one or more successive light pulses emitted by the light source LS. The light pulses are emitted by the light source LS typically at a frequency of a few kHz, wherein the duration t of the light pulses in relation to the period T is small (t / T << 1). Accordingly, the time intervals between the light pulses during which no projection light 40 the lighting system 12 passes.

These long time intervals between the light pulses are used according to the invention, the tilting mirror 32 around one or both tilting axes 34 . 36 to tilt and thereby convert into a second tilted position. In this way, the next light pulse on the target surface 38 a second area 42b lit, like this 2 B illustrated. In the time interval between two consecutive light pulses, moreover, at least one micromirror can 30 of the micromirror array 28 be switched. That on the second area 42b generated light pattern 44b is thus usually the light pattern 44a differ, with the previously in the first area 42a the target area 38 was lit up. The two areas 42a . 42b In this case, preferably adjoin one another directly, so that on the target surface 38 also the light patterns 44a . 44b at least essentially seamlessly merge.

After the second light pattern 44b during one or more light pulses on the target area 38 is generated during another time interval between two successive light pulses of the tilting mirror 32 again at least one of the pivot axes 34 . 36 tilted with the help of the actuators and thus transferred to a third tilt position. This lights up the micromirror array 28 reflected projection light 40 at the next light pulse on the target area 38 a third area 42c with a third light pattern 44c out. If the third light pattern 44c from the second light pattern 44b should also distinguish the micromirrors 30 of the micromirror array 28 at least partially switched.

In the illustrated embodiment, such an adjustment of the tilting mirror is repeated 32 and the micromirror 30 again to the target area 38 in a fourth area 42d a fourth light pattern 44d to create.

After adjustments of the tilting mirror 32 During the time intervals between two consecutive light pulses, the target area thus became over a period of at least four light pulses 38 completely with light patterns 44a to 44d covered. After temporal integration thus results for the resolution of the light pattern over the entire target area 38 a value equal to four times the resolution of one given time from the micromirror array 28 on the target area 38 can be generated.

If several light pulses one of the areas 42a to 42d Moreover, it is possible to illuminate only the micromirrors during the time intervals between these successive light pulses 30 but not the tilting mirror 32 switch. That way, within the ranges 42a to 42d generate different intensity levels after temporal integration. Stand during the exposure of the photosensitive layer 22 A total of 40 light pulses available, so these 40 light pulses evenly on the four areas 42a to 42d be distributed. For every area 42a to 42d There are then 10 light pulses available, representing a gradation of 10 different light intensities within each area 42a to 42d equivalent. This allows using a simple and relatively small digital micromirror array 28 larger contiguous and multi-graded intensity distributions on the target area 38 be generated. This not only allows the precise setting of different illumination angle distributions, but also the generation of field-dependent illumination angle distributions, as described in the already mentioned European patent application with file number EP 13194135.3 is described. The resolution of the light pattern on the optical integrator is so high that different light patterns can be generated on the small light entrance facets of the optical integrator. Since these light patterns are imaged directly on the mask, it is thus also possible to generate field-dependent illumination angle distributions and the dimensions of the illumination field 16 via the control of the micromirror array 28 influence.

The 3 shows in a to the 2a to 2d a schematic perspective view of another embodiment of an inventive illumination system, wherein in the light path between the micromirror array 28 and the target area 38 a lens indicated as a single lens 46 is arranged. The objective 46 forms the micromirror array 28 on the target area 38 from.

Furthermore, in the 3 actuators 48 . 50 for the tilting mirror 32 and a control device 52 shown. The control device 50 controls the light source LS, the actuators 48 . 50 and the micromirror array 28 and thus provides the necessary synchronization between the light pulses generated by the light source LS on the one hand and the adjustment of the tilting mirror 32 and the micromirror 30 for sure.

With the help of in the 30 an arrangement can be realized in which the light pattern on the target surface are interlocked, as shown in the 4 is illustrated. In the 4 There are four light distributions on the micromirror array above 28 shown at four different times; the simplicity has been assumed here that the micromirror array 28 only 4 micromirrors 30 having. One recognizes that the micromirrors 30 through larger non-reflective areas 54 are separated from each other. In this embodiment, the dimensions of the micromirrors 30 along a reference direction, for example, the X direction, are not larger than the distances between adjacent micromirrors 30 along this reference direction.

As the 4 illustrated in the middle, in this embodiment, the actuators 48 . 50 for the tilting mirror 32 so from the controller 52 controlled that the light pattern 44a to 44d be interlocked. The areas 42a to 42d after each adjustment of the tilting mirror 32 on the target area 38 are illuminated, are only about the width of the image 30 ' a micromirror 30 offset from each other, causing the envelopes of the areas 42a to 42d each overlap significantly. By the gradual pivoting of the areas 42a to 42d becomes the target area after temporal integration 38 fully lit, as shown below 4 is indicated.

The tilted in the "off position" and therefore black indicated micromirrors 30 in the light patterns 44a and 44d limitations in the temporally integrated intensity distribution, as shown below 4 shown at their corners to each other. By pivoting the light pattern with the help of the tilting mirror 32 can thus be achieved that even with an image of the micromirror array 28 with the help of the lens 46 on the target area 38 an intensity distribution can be generated which is free of images of the non-reflective areas 54 is.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 1262836 A1 [0004]
• US 2006/0087634 A1 [0004]
• US 7061582 B2 [0004]
• WO 2005/026843 A2 [0004]
• WO 2010/00668 A1 [0004]
• WO 2012/100791 A1 [0005]
• EP 13194135 [0005, 0006, 0045]
• US 6624880 B2 [0009]
• US 2011/0134407 A1 [0009]
• US 7957055 B2 [0009]
• US 2011/0240611 A1 [0010]

Claims (13)

Illumination system of a microlithographic projection exposure apparatus ( 10 ), with a) a light source (LS), which is set up to generate a sequence of light pulses, b) a target surface ( 38 ), c) an array arranged in the light path between the light source and the target surface ( 28 ) of optical elements ( 30 ) which are digitally switchable between two switch positions, d) an adjustable light deflection optics arranged in the light path between the array and the target surface ( 32 ), which is adapted to receive incident light ( 40 ) with different deflection angles, and e) a control device ( 52 ), which is adapted to control the light deflection optics so that a change in the deflection angle between two successive light pulses takes place. Illumination system according to Claim 1, in which the control device ( 52 ) is adapted to at least one of the optical elements ( 30 ) of the array ( 28 ) to switch between two light pulses. Illumination system according to Claim 1 or 2, in which the light deflection optics ( 32 ) and the control device ( 52 ) are arranged so that one before a change of the deflection angle of the array ( 28 ) on the target surface ( 38 ) illuminable first light pattern ( 44a to 44d ) with a second pattern of light which can be illuminated on the target surface after a change of the deflection angles ( 44a to 44d ) overlaps. Illumination system according to Claim 3, in which the first light pattern ( 44a to 44d in 2a to 2d ) and the second light pattern ( 44a to 44d in 2a to 2d ) are arranged on the target field such that envelopes of the two light patterns do not overlap. Illumination system according to Claim 3, in which the first light pattern ( 44a to 44d in 4 ) and the second light pattern ( 44a to 44d in 4 ) are interlaced. Illumination system according to Claim 5, in which the distance between two adjacent optical elements ( 30 ) of the array ( 28 ) along a reference direction (X) is greater than the maximum dimension of the two optical elements ( 30 ) along the reference direction. Illumination system according to one of the preceding claims, in which in light path between the array ( 28 ) and the target area ( 38 ) a lens ( 46 ) which images the array onto the target surface. Method for operating a lighting system ( 12 ) of a microlithographic projection exposure apparatus ( 10 ), comprising the following steps: a) directing a sequence of light pulses onto an array ( 28 ) of optical elements ( 30 ), which are digitally switchable between two switch positions, wherein the array has a target area ( 38 ) illuminates; b) deflecting the light directed from the array onto the target surface by different deflection angles through an adjustable light deflection optic arranged in the light path between the array and the target surface ( 32 ), wherein a change in the deflection angle between two successive light pulses takes place. Method according to Claim 8, in which at least one of the optical elements ( 30 ) of the array ( 28 ) switches between two light pulses A method according to claim 8 or 9, wherein a prior to a change in the deflection angle of the array ( 28 ) on the target surface ( 38 ) illuminable first light pattern ( 44a to 44d in 2a to 2d ) with a second pattern of light which can be illuminated on the target surface after a change of the deflection angles ( 44a to 44d in 2a to 2d ) overlaps. The method of claim 10, wherein the first light pattern and the second light pattern are arranged on the target field such that envelopes of the two light patterns do not overlap. Method according to Claim 10, in which the first light pattern ( 44a to 44d in 4 ) and the second light pattern ( 44a to 44d in 4 ) are interlaced. Method according to Claim 12, in which the spacing between adjacent optical elements ( 30 ) of the array ( 28 ) along a reference direction (X) is greater than the maximum dimension of an optical element along the reference direction.

Images